If $${x^2} + {y^2} = {a^2}$$, then $$\int\limits_0^a {\sqrt {1 + {{\left( {{{dy} \over {dx}}} \right)}^2}} dx = } $$
- A2$$\pi a$$
- B$$\pi a$$
- C$${1 \over 2}\pi a$$
- D$${1 \over 4}\pi a$$
✓ Correct answer: C
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